The present invention concerns a process for producing metal strips provided with raised reinforcing portions of substantial height, by rolling, and the apparatus for carrying out the process, that is to say, the corresponding rolls of suitable shape.
For the economic production of boarding and planking or bridge flooring, it has long been known to use metal sheets whose moment of inertia is reinforced by ribs. For that purpose, panel elements of aluminum of a plate configuration which are reinforced on one face by longitudinal ribs are currently produced by an extrusion process. Extrusion is even used to produce box elements of closed section in the direction of the extrusion process. However, extrusion is still a process which involves certain disadvantages.
A process involving rolling between rolls comprising hollowed-out channels or patterns is currently used to produce strips of aluminum comprising corresponding patterns of a raised configuration, which strips can then be cut up into metal sheets of the desired format. However, the above-mentioned patterns such as those on teardrop-type metal sheets are of small thickness, of the order of a millimeter. They have a decorative or non-slip effect but they have virtually no influence on the bending strength of the metal sheets or plates.
In order to facilitate the movement of the rolled metal into the cavities in the roll, to avoid incipient rupture at the base of the raised patterns on the metal sheet, the recesses in the roll connect to the surface of the roll without a sharp angle, in most cases by way of a rounded or at least chamfered or bevelled surface. Generally, in the prior art, the side faces of the recesses in the roll are inclined surfaces which converge towards the bottom of the recess and which are not perpendicular to the surface of the roll.
In another respect, calculation shows that, in order to produce patterns of substantial thickness on the strip, it is desirable to have a substantial coefficient of friction between the metal and the roll, and also a substantial reduction in the thickness of the metal on respective sides of the pattern. Those are difficult and burdensome conditions in regard to the rolling operation. Now, it has been found that, if it was not desirable to have sharp edges at the junction between the surface of the roll and the surface of the recesses but, in accordance with the prior art, such junction rather comprises rounded or at least chamfered connecting portions, it was in contrast undesirable, contrary to the generally accepted ideas, for the inclined convergent surfaces to be continued to the bottom of the recesses (normal taper or relief). In fact, the metal which goes to fill the recesses in the roll is retarded in its lateral displacement by friction against the inclined surfaces of the recesses. It was found that much better results are obtained by providing the rolls with recesses in which, except for the mouth opening thereof, the side surfaces are perpendicular to the axis of the roll or even inclined in a `counter-taper configuration`, that is to say, recesses with a dovetail section which increases in width towards the bottom of the recess. As the metal penetrates into such recesses, it is no longer retarded by frictional engagement against the side wall surfaces of the recesses. However, the outward flare of the recesses towards the bottom thereof may only be slight. The `counter-taper` inclination (.alpha.) of the side wall surfaces with respect to the lines perpendicular to the axis of the roll must be limited to an angle of the order of 5.degree., in order to limit the possible expansion of the metal which is urged into the recesses and to permit the patterns which are formed in a raised configuration on the strip to be removed from the recesses in the roll without too much difficulty in the subsequent stage. The above-mentioned form of the recesses in the rolling rolls, being flared slightly towards the bottom of the recesses, has been found to be a particularly attractive proposition for producing metal sheets or plates which are longitudinally ribbed in the direction of rolling, in which case the recesses are then in the form of circular channels or passes.
Obviously, the high ribs, which have parallel side faces, stick strongly in the recesses or channels in the roll. That phenomenon is further accentuated when the recesses or channels in the roll have side faces of `counter-taper` type which produce in the strip ribs of dovetail cross-section, in the axial plane of the rolls.
On issuing from the roll, a substantial pulling force has to be applied to the strip in order to detach it from the roll and to prevent it from being wound round on to the roll. The head portion of the ribs or at least the central portion thereof if the material has not filled the entire volume of the roll grooves is, as it were, rolled laterally when they leave the reduced exit orifice of the channels in the roll. Contrary to what might have been thought, there is no tearing or wrenching of a rib, nor even incipient rupture at the root of the ribs, provided obviously that the counter-taper inclination of the side faces of the channels in the roll is slight, that is to say, of the order of 5% with respect to planes that are perpendicular to the axis of the roll. The pulling force to be applied to the rolled strip in order to detach it from the channel-bearing roll, if it is expressed with respect to the cross-section of the strip, may be greater than 40 N/mm.sup.2. For aluminum and alloys thereof, it is generally of the order of 40 to 50 N/mm.sup.2.
However, having regard to the power levels available on present-day cold rolling mills, the raised portions of substantial height can be produced only on relatively soft metals or alloys which are sufficiently ductile in the cold condition; aluminum alloys in the annealed state or similar states, in respect of which the eleastic limit is lower than or equal to 200 MPa, are particularly suitable for the process according to the invention. In another respect, certain geometrical conditions are required in regard to filling the recesses in the roll, the depth (p) thereof being generally greater than their width (1), and they must be separated by a distance (d) which is between 0.5L and 3L. Below 0.5L, the mechanical strength of the land between two channels or passes becomes insufficient; above a value of 3L, the channels are not sufficiently filled by the metal.
It is preferable that the overall rolling effect imposed on the strip, as measured by ##EQU1## wherein S is the initial cross-section and s is the final cross-section of the product, is high, being generally greater than 60.